Friction stir welding - tool developments (February 2001)

TWI Ltd

Paper presented at the Aluminum Joining Symposium during the 2001 TMS Annual Meeting, 11-15 February 2001, New Orleans, Louisiana, USA

Abstract

By any standard the industrial adoption of friction stir welding as the preferred joining technique for a range of aluminium alloys represents a remarkable progress of technical development. Furthermore, a range of non-ferrous and ferrous materials has also been shown to be readily welded by FSW in the laboratory.

The design of the tool is the key to the successful application of the process to a greater range of materials and over a wider range of thickness. A number of different high performance tool designs have been investigated. This paper describes recent developments using these enhanced tools from the perspective of existing and potential applications.

Introduction

Friction stir welding (FSW) was invented and developed at TWI ^[1] . It allows metals, including aluminium, lead, magnesium, steel and copper, to be welded continuously ^[1,18] . Many alloys, which are regarded as difficult to weld by fusion processes, may be welded by FSW. The process has already made a significant impact on the aluminium producing and user industries.

Fig.1. Schematic showing the basic principle of the FSW process

A non-consumable rotating tool is employed of various designs, which is manufactured from materials with superior high temperature properties to those of the materials to be joined. Essentially, the probe of the tool is plunged into the abutting faces of the workpieces, while the tool is rotated, thereby generating frictional heat. This heat creates a softened plasticised region (a third-body) around the immersed probe and at the interface between the shoulder of the tool and the workpiece. The shoulder provides additional frictional treatment of the workpiece, as well as preventing plasticised material from being expelled from the weld region. The strength of the metal at the interface between the rotating tool and the workpiece falls to below the applied shear stress as the temperature rises, so that plasticised material is extruded from the leading side to the trailing side of the tool. The tool is then steadily moved along the joint line giving a continuous weld.

Tool design

Early on in the development of FSW it was realised that the detailed form of the welding tool was critical in achieving sound welds with good mechanical properties. Some of the most recent developments at TWI are those of the Whorl ^TM and MX Triflute ^TM tools ^[19,21] . In general terms, the tool comprises a shoulder and a probe. The shoulder compresses the surface of the workpiece and contains the plasticised weld region. Heat is generated on the surface by friction between the rotating shoulder and the workpiece surface and, when welding comparatively thin sheets, this is the main source of heat. However, as the workpiece thickness increases, more heat must be supplied by friction between the rotating probe and the workpiece. In addition to this, the main functions of the probe are to ensure sufficient working of the material at the weld line and to control the flow of the material around the tool to form a satisfactory weld. The Whorl ^TM tool is shown in Fig.2 and an illustration of the MX Triflute ^TM is shown in Fig.3.

 

Fig.2. Prototype Whorl TM tool superimposed
on a transverse section of a weld

Fig.3. MX Triflute TM

Essentially, the probe for both tools is shaped as a frustum that displaces less material than a cylindrical tool of the same root diameter. (Typically, the Whorl ^TM reduces the displaced volume by about 60%, while the MX Triflute ^TM reduces the displaced volume by about 70%). Provided a certain minimum probe tip diameter is maintained, the frustum shape requires less effort to traverse it through the plasticised material than a cylindrical probe. This gain in welding performance is due to the reduction in probe volume and its design features ^[22] . It should also be noted that all re-entrant features, especially the change in section between the shoulder and the probe, are well radiused in order to reduce stress concentration and thereby fracture of the tool.

The Whorl ^TM tool shape ensures that the lower surface of the helical ridge provides a clear downward augering force, with less interference from the next ridge below. The core of the probe need not run parallel with the helical ridge, nor does the ridge have to be of uniform pitch. In this respect, the helical ridge is not a simple external thread, which has to engage with an internal thread, but is essentially an auger that is immersed in the plasticised medium which the tool creates.

To enable more effective flow of the plasticised material, it is preferred that the distance between each ridge is greater than the thickness of the ridge itself. The Whorl ^TM concept provides for probe cross-sections that are circular, nominally oval, flattened, or re-entrant. In this way, the probe displacement volume is less than its volume of rotation. The combined use of a helical ridge and re-entrant features means that these types of probe further enable the easier flow of plasticised material. In addition, the inclination of the continuous spiral ridge can be designed and manufactured to suit the material being welded. Variation in the inclination of the spiral ridge allows adjustment to the degree of stirring and the downward movement of the plasticised material.

For Whorl ^TM and MX Triflute ^TM probes, the downward thrust and rotation (as viewed from underneath the shoulder) are in a clockwise direction for a right hand spiral, and vice versa. Friction stir welding probes are subjected to cyclic bending (similar in nature to a rotating cantilever) and torsion loads. Both probes are more uniformly stressed during welding and allow for a more efficient flow path than the conventional cylindrical pin type probe. The probes generally have a profiled or threaded surface to facilitate the downward augering effect. Preferably, they have an odd number of equally spaced flutes to maintain maximum bending strength opposite to any re-entrant feature. Figure 4 shows cross sections of probe profiles.

Fig.4. Illustration of three and four fluted probe profiles of the same cross-sectional area

A coarse helical ridge around the Triflute ^TM lands is also employed. This additional helical feature is employed to further reduce the tool volume (and therefore aid material flow), and to help break up and disperse the surface oxides at the weld line. Moreover, the re-entrant helical flutes and thread features used on these probes increase the surface area of the probe. This effectively means that the interface between the probe and the plasticised material is also increased, thereby increasing heat generation.

It is believed that the major factor that determines the superiority of the Whorl ^TM and MX Triflute ^TM probes over the conventional cylindrical pin type probe (especially for thick plate welding) is the ratio of the volume of the probe swept during rotation to the volume of the probe itself. It is this ratio of the 'dynamic volume' to the static volume that is important in providing an adequate flow path. Typically, this ratio for probes having similar root diameters and probe length was 1.1:1 for conventional pin probes, 1.8:1 for Whorl ^TM probes and 2.6:1 for MX Triflute ^TM probes (when welding 25mm thick plate).

Shoulder rofiles

Shoulder profiles are also used and these are designed to suit different materials and conditions as necessary. ^[19,20,22] These shoulder profiles improve the coupling between the tool shoulder and the workpiece by entrapping plasticised material within special re-entrant features. This essentially, provides like-to-like frictional contact and improved weld closure by helping prevent plasticised material from being expelled. Examples of different shoulder profiles are shown in Fig.5.

Fig.5. Tool shoulder geometries, viewed from underneath the shoulder

 

Finite element modelling of FSW probes

FSW tools are heavily loaded during welding. This creates stresses that could potentially break the tool through fatigue (cyclic loading) or overload. Experimental measurements of stresses are difficult to achieve, but finite element analysis (FEA) provides a possible method of determining the stress distributions and can be used to compare the likely integrity of various tool designs.

TWI has used FEA to make comparisons of tool designs. The comparisons were made on the basis of uniform pressure loads applied to the faces of the tool to create pure torsion, pure bending and combinations of the two loading modes. Tools of differing designs were compared using peak stresses for the same level of applied torque and/or bending moment (an example predicted stress distribution is shown in Fig.6). Further work is needed to improve these calculations. It would be advantageous to predict the welding torque for different tool designs. This would allow the integrity of tools to be compared on the basis of the loading each would experience during welding. TWI is currently investigating a range of techniques for this purpose.

Fig.6. Predicted stresses in a helical FSW tool under a combined torsion, bending and shear loading

 

Metallographic examination

Aluminium alloy plates of thicknesses 1mm to 50mm have been successfully friction stir welded in one pass and a 75mm thick FSW weld in 6082 T6 aluminium alloy plate is shown in Fig.2. The latter was achieved with a Whorl ^TM tool in two passes, each giving about 38mm penetration.

In order to reveal the three-dimensional nature of the weld nugget features a weld specimen was machined and polished to produce a number of taper sections. Figure 7 shows the position of the angles taken.

Fig.7. Position of taper section taken

Figure 8 a, b, c, d, e & f, show a series of macrostructural features as revealed from a series of tapers sections prepared from the same weld specimen. These were produced from 25mm thick 6082 - T6 condition aluminium alloy.

Fig.8. Macrostructural features in the nugget region. Weld produced in 25mm thick 6082 T6 aluminium alloy using a Whorl ^TM tool, at a weld travel rate of 4mm/sec (240mm/min)

Fig.8a) Transverse macrosection at 10 degree taper

Fig.8b) Transverse macrosection at 20 degree taper

Fig.8c) Transverse macrosection at 30 degree taper

Fig.8d) Transverse macrosection at 40 degree taper

Fig.8e) Transverse macrosection at 60 degree taper

Fig.8f) Transverse macrosection at 70 degree taper

These macrosections reveal well-defined flow contours within the weld nugget region, and confirm previous observations regarding the so-called onion feature within the weld nugget ^[6] .

The composition of the nugget is unchanged from that of the parent material and there is no apparent segregation of alloying elements.

Encouraging results and good performance have been achieved by using the MX Triflute ^TM type tools to make single pass welds in a number of materials, from 6mm to 50mm in thickness. A transverse macrograph, Fig.9, shows a well-defined heat affected zone (HAZ) that surrounds the weld nugget in 6mm thick 5083 0 condition aluminium alloy.

 

Fig.9. Transverse section from weld produced in 6mm thick 5083 O condition aluminium alloy using a MX Triflute ^TM tool, at a weld travel rate of 9mm/sec (540mm/min) ()

Approval of the FSW process

The potential application of, and confidence in, the process is demonstrated by the fact that surveying bodies such as Germanischer Lloyd, Det Norske Veritas, Registro Italiano Navale and TUV Suddeutschland have given approval to the welding procedures for specific applications.

Concluding remarks

The tool is fundamental to the successful implementation of FSW. TWI has shown that the tool design can be improved through the practical implementation of some fundamental concepts. These concepts can be further fine-tuned by computer-based stress analysis of the complex tool geometry.

Acknowledgements

The authors wish to thank K I Johnson, M F Gittos, P B Fielding, C Schnieder, P Temple-Smith, L Signorini, P L Threadgill and S W Kallee for their support and advice.

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